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When government grows, prosperity shrinks. So does freedom. With those powerfully simple truths in mind, Rep. Jeb Hensarling, Texas Republican, and Rep. Mike Pence, chairman of the House Republican Conference, on March 3 introduced one of the most intriguing constitutional amendment proposals in years. Their Spending Limit Amendment merits great attention and with a few tweaks will deserve enthusiastic support.

Except in times of declared war or when two-thirds of each house of Congress decides otherwise, the amendment would limit “total annual outlays” of the federal government (except repayment of principle of the national debt) to “one-fifth of economic output” of the country. Mr. Pence and Mr. Hensarling, along with original co-sponsor Rep. John Campbell, California Republican, note that federal spending of 20 percent of gross domestic product (GDP) has been the historical average since World War II, and spending didn’t breach 23 percent in a single year for more than four decades. Under the current spending explosion, however, outlays have risen from exactly 20 percent to a near-astronomical 24.7 percent in just two years, with the trend line heading toward 40 percent over the next two decades.

By foisting mind-boggling amounts of debt onto generations yet unborn, this spending is so unsustainable as to be immoral. It’s also completely counterproductive in the short term. “You cannot have unlimited government and unlimited opportunity,” Mr. Hensarling said Thursday on a media conference call. He’s right. So was Mr. Pence when he repeatedly emphasized that limited government and minimal debt are essential elements to national security. That’s because domestic spending sprees crowd out available resources for security and because overspending often puts us in hock to foreign governments that buy our debt.

The fact is that when 1995’s new Republican Congress began rescinding (taking back) federal spending that already had been approved and then held the line on spending for the next three years while also reforming welfare, the economy boomed. Spending was 21 percent of GDP in 1994, and it was held in succeeding years to percentages of 20.7, 20.3, 19.6, 19.2, 18.7 and 18.4. As spending discipline increased, GDP grew during that period at a steady and superb clip of an average 4 percent per year, and the national unemployment rate improved steadily from 6.1 percent to 4 percent.

One drawback is that the amendment as drafted leaves it to Congress to pass “enabling legislation” to put the limit into practice. For example, the amendment does not explain how “one-fifth of economic output” is to be measured. Mr. Hensarling suggests using the average GDP for the previous five years. But lawmakers could instead define it as some sort of “projected” GDP for the coming year and then fudge the projections to give themselves more wiggle room. To avoid that, the amendment should define its goals with more specificity.

Nitpicking aside, the amendment makes a ton of sense. Members of Congress who won’t sign onto it ought to be held to account.